The present invention relates generally to power amplifier circuits, and more specifically to a broadband power amplifier circuit providing wide bandwidth with low distortion.
The development of CAble TeleVision (CATV) system networks and other broadband networks has created an ever-increasing need for wide bandwidth power amplification. A typical CATV network includes a headend, a trunk system comprising a plurality of trunk cables, a distribution system comprising pluralities of nodes and feeder cables, and a plurality of subscriber drop cables. The headend typically receives all of the signals carried by the CATV network (e.g., from a satellite, a broadcast network, or another CATV network), and transmits the signals as a broadband signal to the distribution nodes via the trunk cables. The distribution nodes then distribute the broadband signal to the subscriber drop cables via the feeder cables. For example, each feeder cable may have a number of broadband signal taps sufficient to connect the feeder cable to hundreds or even thousands of subscriber drop cables. Each broadband signal tap typically provides a portion of the broadband power to a respective subscriber drop cable, and transmits the remaining broadband power to the next signal tap on the feeder cable. The subscriber drop cable then provides the broadband signal directly to the subscriber""s equipment, which may comprise a suitable converter or television receiver.
In the typical CATV network, the headend includes a plurality of broadband power amplifier circuits configured to transmit the respective broadband signals to the distribution nodes for subsequent distribution to the subscriber drops. A conventional broadband power amplifier includes active devices such as Bipolar Junction Transistors (BJTs) implemented in a hybrid circuit and fabricated using silicon technology.
However, silicon fabrication processes have drawbacks in that they have generally failed to keep pace with increasing demands for wide bandwidth power amplification with low distortion. For example, in order to increase the bandwidth of silicon BJT devices, physical dimensions of the devices may be reduced to minimize electrical parasitic effects. Such reductions in device dimensions can limit the internal power dissipation capability of the devices, which in turn can increase internal device temperatures and cause higher levels of distortion in amplified broadband output signals. Higher distortion levels can necessitate the use of increased numbers of broadband power amplifiers at the headend to achieve a desired performance at the subscriber locations, thereby increasing the cost and complexity of the CATV network.
In order to overcome at least some of the drawbacks of broadband power amplifier circuits fabricated using silicon technology, conventional broadband power amplifiers have alternatively been fabricated using a Gallium Arsenide (GaAs) MEtal Semiconductor Field Effect Transistor (MESFETs) process. However, such broadband power amplifiers comprising GaAs MESFET devices have drawbacks in that they may also fail to achieve desired levels of performance. For example, although GaAs MESFET devices are typically very fast and highly stable over wide temperature ranges, broadband power amplifiers comprising GaAs MESFET devices may have distortion levels that exceed those of broadband power amplifiers comprising silicon BJT devices at high output power levels.
It would therefore be desirable to have a broadband power amplifier circuit that can be employed in CATV system networks and other broadband networks. Such a broadband power amplifier circuit would be configured to provide wide bandwidth with low distortion across an increased range of output power levels.
In accordance with the present invention, a broadband power amplifier circuit is disclosed that provides wide bandwidth with low distortion across an increased range of output power levels. Benefits of the presently disclosed broadband power amplifier circuit are achieved by fabricating the circuit using a Gallium Arsenide (GaAs) pseudomorphic High Electron Mobility Transistor (pHEMT) process, and by configuring the circuit so as to enhance its internal power dissipation capability.
In one embodiment, the broadband power amplifier circuit comprises a GaAs pHEMT Monolithic Microwave Integrated Circuit (MMIC) including two (2) serially coupled cascode amplifiers in a first half of the circuit, and two (2) serially coupled cascode amplifiers in a second half of the circuit. The broadband power amplifier circuit further comprises a first transmission line balun configured to receive a single-ended Radio Frequency (RF) or microwave input signal at an input of the circuit, and provide first and second balanced low level signals to the two (2) cascode amplifiers of the first and second circuit halves, respectively; and, a second transmission line balun configured to receive first and second balanced high level signals generated by the two (2) cascode amplifiers of the first and second circuit halves, respectively, and provide a single-ended amplified broadband output signal at an output of the circuit.
The two (2) serially coupled cascode amplifiers of the first and second halves of the broadband power amplifier circuit include respective input GaAs pHEMT devices and respective output GaAs pHEMT devices. Predetermined DC bias currents are applied to the respective drains of the output GaAs pHEMT devices via inductors and returned to ground potential via resistors coupled to the respective sources of the input GaAs pHEMT devices. Predetermined DC bias voltages are applied to the respective gates of the GaAs pHEMT devices and set by at least one resistive voltage divider circuit. In this way, the broadband power amplifier circuit is biased to operate in a Class-A, push-pull cascode arrangement.
By fabricating the broadband power amplifier circuit using the GaAs pHEMT process, gate-to-drain breakdown voltages of the active GaAs pHEMT devices are increased. As a result, the amplified broadband output signal generated by the broadband power amplifier circuit has high power capability with low distortion. Moreover, by biasing the respective GaAs pHEMT devices in the cascode amplifier sections using the resistive voltage divider circuit to allow for successive drops in the applied DC bias voltage levels, the internal power dissipation capability of the circuit is enhanced. As a result, internal device temperatures are reduced and lower levels of distortion are achieved.
Other features, functions, and aspects of the invention will be evident from the Detailed Description of the Invention that follows.